The present invention relates to hydrodynamic reversing transmissions for automotive vehicles, especially for heavy duty dump trucks and analogous conveyances. More particularly, the invention relates to improvements in hydrodynamic reversing transmissions of the type disclosed in German Pat. No. 1,178,101 (note particularly FIGS. 1 and 2 of this patent). Attention is further invited to commonly owned U.S. Pat. No. 4,099,426 granted July 11, 1978 to Keller et al. whose disclosure is incorporated herein by reference.
German Pat. No. 1,178,101 discloses a transmission which is designed for use in rail-mounted vehicles, particularly in locomotives for passenger or freight trains as well as in locomotives for shunting or switching service in marshalling yards. The patented transmission comprises a hydrodynamic torque converter in the forward drive train and a hydrodynamic torque converter in the reverse drive train. As a rule, the two torque converters are used alternately, i.e., one is permitted to receive pressurized working fluid while the working fluid is allowed to escape from the chamber of the other torque converter and vice versa. It is also possible to operate in the so-called counterbraking range in which the turbine of one of the torque converters operates in the negative direction, i.e., to allow one of the torque converters to receive pressurized fluid while the other torque converter is drained. Each such torque converter constitutes a starting torque converter, i.e., they are used for operation in the lower speed range. For traction operation in the higher speed range, the transmission of the German patent comprises a clutch, preferably a hydrodynamic clutch.
When the transmission of the German patent is installed in a locomotive for use in a marshalling yard, the vehicle is caused to change the direction of its movement by evacuating one of the starting torque converters and by admitting working fluid to the chamber of the other torque converter. When the locomotive which embodies such transmission is used for long-distance service to pull passenger or freight trains, the locomotive is equipped with an additional mechanical reversing transmission which is used to change the direction of travel of the locomotove and renders it possible to use the clutch during travel in either direction. A drawback of such conventional transmissions is that, for hydrodynamic braking with one of the two torque converters in the counterbraking range, the energy consumption of the pump wheel in the torque converter is relatively high. In other words, the torque converter which is used to furnish a braking action consumes energy which is furnished by the engine at the time when such torque converter serves to brake the vehicle. However, the just mentioned drawback is not overly serious in conventional transmissions because hydrodynamic braking is resorted to only in the course of the shunting or switching service, i.e., the duration of braking operation is short or very short.
The situation is different when a hydrodynamic reversing transmission of the above outlined character is used to furnish longer-lasting braking actions, for example, when a vehicle is driven on a downwardly sloping road. Such situation does not arise often in connection with rail-mounted vehicles but is much more likely to be encountered with road vehicles, especially heavy duty vehicles of the class including dump trucks or the like. As a rule, such vehicles cover long distances without ever moving in reverse, i.e., the preferred direction of travel of such vehicles is the forward direction. Therefore, it is not necessary to provide a separate or additional mechanical reversing transmission in or for such types of vehicles. It suffices to provide the transmission which is used therein with means (e.g., a torque converter) for effecting a relatively slow movement in the rearward direction. The clutch is used only when the vehicle travels in the preferred (forward) direction and while the vehicle is driven at a relatively high speed.